Groundwater contamination by nitrates associated with intensive potato culture in Québec

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Abstract

In rural areas, groundwater contamination by nitrates is a problem related to the spreading of organic and chemical fertilizers by farmers and, to some extent, to effluents from domestic sewage systems. Health effects of groundwater contamination by nitrates have been assessed several times and may lead to important consequences for infants. Following pressures from citizens in 1990, a survey of well water quality around potato fields of the Portneuf county (Québec) found that nitrate contamination was frequently above the 10 mg-N/l standard. Because this first survey was limited to areas of intensive potato culture, it was not possible to evaluate the real impact on the groundwater quality for the whole county and the subsequent public health intervention was spread over the entire region. A second survey was carried out in 1995 to reevaluate the situation using random sampling methods. This latter study took into account drinking water habits of the population, the relative importance of potato culture as a source of nitrogen loading, the effects of soil types, and waste-water disposal systems as well as land use on nitrate concentration in private well water. The data analysis was carried out by combining GIS and statistical methods to test hypotheses about the spatial relationship linking measured nitrate concentrations with their immediate environment. This paper presents the major findings from this second study which confirm the impact of intensive potato culture on groundwater nitrate concentrations, mainly localized in sandy soil areas within 2 km of fields. Finally, it illustrates the usefulness of GIS to focus public health interventions.

Introduction

Groundwater contamination by nitrates is a worldwide problem mainly related to the important use of fertilizers in intensive agriculture (World Health Organization, 1985, Kenney, 1986, Power and Schepers, 1989, Bouchard et al., 1992). Since private use of groundwater is usually not regulated, it is difficult to get a clear idea about the temporal variation of its quality. However, considering reports from several industrialized countries, there appears to be an increasing trend in the contamination of groundwater by nitrates, related to intensification of agriculture, at least in Europe and North America (ECETOC, 1988, HED, 1988).

Health effects of nitrate contamination have been assessed several times during the last decade (Fan et al., 1987, ECETOC, 1988, Gangolli et al., 1994, Levallois and Phaneuf, 1994, National Research Council, 1995, Fan and Steinberg, 1996). While infant methemoglobinemia has been recognized for a long time as a possible consequence of groundwater contamination by nitrates (Walton, 1951), there is still discussion regarding the possible overprotection afforded by the current water quality guidelines (ECETOC, 1988). No methemoglobinemia has been reported in North America in infants drinking water with nitrate concentrations under the 10 mg-N/l1 quality guideline, and most of the cases of methemoglobinemia with nitrate levels between 10 and 25 mg-N/l have been reported in infants with diarrhea, a condition favoring important endogenous nitrate formation (National Research Council, 1995, Fan and Steinberg, 1996). It is also probable that present intake of vitamin C protects infants from the formation of methemoglobin (Fan and Steinberg, 1996). In any case, the 10 mg-N/l (or 44.3 mg/l of nitrate) constitutes a well accepted level for public health intervention (National Research Council, 1995). The target is to inform parents to protect their infants by preparing milk bottles with water respecting this guideline. Other health effects associated with nitrates exposure are uncertain. While reproductive and cancer effects are still under study, they seem unlikely if the 10 mg-N/l guideline is respected (National Research Council, 1995, Fan and Steinberg, 1996). In this context we present our experience with the use of environmental data as input for public health interventions.

In 1990, following pressures from citizens, a well water quality survey was carried out in the Portneuf county localized in rural Quebec. The citizens were mainly worried about the use of pesticides in the intensive culture of potato which is present in some very localized sectors of the Portneuf county. The survey done during the fall of 1990 on 70 private wells located in areas of intensive potato culture found few pesticides in groundwater but a generalized contamination by nitrates: 63% of the wells had nitrate levels above 3 mg-N/l and 41% over the 10 mg-N/l standard, with a maximum of 33 mg-N/l (Paradis et al., 1991). Contamination was particularly important in sand point wells: 24/28 (85%) with nitrate levels above 3 mg-N/l and 18 (57%) above the 10 mg-N/l level. Since such a situation could generate significant health consequences, the public health department of the region decided to inform all parents of newborns as well as pregnant women from all over the county about the problem and advised them to check the quality of their well water before using it to prepare bottles for infants. Physicians were also informed of the possibility of infant methemoglobinemia and a reprint of a recent report on a death from methemoglobinemia in South Dakota (Johnson et al., 1987) was sent to all general practitioners and pediatricians of the county.

Since the relation with potato culture was quite obvious, information was also provided to potato growers in order to convince them to change their cultural practices to reduce groundwater contamination by nitrates. Because the 1990 survey was limited to areas of intensive potato culture, it was difficult to evaluate the real impact of this very localized and disseminated culture on the groundwater quality of the whole county. However, the first public health intervention following this survey was focused on the whole county by assuming that all the groundwater of the county was at risk for nitrate contamination.

A second survey was carried out in 1995 in order to reevaluate the situation all over the county taking into consideration: firstly the drinking water habits of the population and the quality of groundwater with regard to nitrates; secondly, the relative importance of potato culture as a source of nitrate contamination in comparison to other activities; and thirdly, the effects of environmental context (land use, soil types, topography, etc.) over the migration of nitrates to the aquifer.

Section snippets

Methodology

The Portneuf county is a low density rural area of 52 500 inhabitants, located south west of Québec City. Agriculture, forestry and paper mills are the main economic activities of the region and most of the people live in small towns. Potato culture is not dominant (nearly 6% of cultivated areas) and is dispersed in space following the distribution of Morin (sand), Sorel (coarse sand with loam), Uplands and Saint-Thomas (sand and loam) soil types. Other agricultural activities are related to

Results

Among the 222 people interviewed by phone, 152 (68.5%) reported drinking only tap water, 50 (22.5%) were drinking bottled water (with or without tap water) and 20 (9%) tap water with home treatment. No significant difference in drinking water patterns was found according to the type of well (Table 1). Of the 70 people rejecting regular tap water (e.g. without any treatment), 58 (82.9%) reported that the main reason for not drinking tap water was its organoleptic properties. Only eight (11.4%)

Discussion

The problem of groundwater contamination by nitrates appears to be worsening at the end of the 20th century. Since acute health effects in infants exposed to high doses of nitrates are clearly established and also because chronic exposure could possibly lead to deleterious effects in adults, there is a need to precisely evaluate cases of contamination by nitrates in order to propose adequate preventive measures and public health interventions.

In the Portneuf county, the intensive potato culture

Conclusion

The present study confirms that intensive potato culture on sandy soil could have serious impacts on the quality of groundwater. While the present findings are probably less alarming than what was found in intensive agricultural areas of Europe or USA (World Health Organization, 1985, ECETOC, 1988, Bouchard et al., 1992), they are a signal that such a practice could have long-term impacts on the quality of groundwater. Since the movement and renewal of the groundwater is very slow, it is

Acknowledgements

This study was funded by a grant from the Tri-Council Secretariat of the Eco-Research Program (Part of Canada's Green Plan; 922-94-0015).

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